Heated conduit

ABSTRACT

A conduit comprising a pipe, an electrical heater extending along the length of the pipe, and a thermally insulating jacket extending around the pipe and heater. The pipe comprises a plastic inner tube. The plastic inner tube is received within a deformable metal tube such that the plastics inner tube and deformable metal tube form a composite double-walled structure.

The present invention relates to a conduit comprising a pipe, anelectrical heater extending along the length of the pipe, and athermally insulating jacket extending around the pipe and heater.

Coilable conduits incorporating heating cables and insulating jacketsare known. In a typical such conduit, a pipe is formed from a metal suchas copper or stainless steel. A cable is run along the length of thepipe and secured in position on the pipe by a strapping film helicallywound around the pipe and cable assembly. An insulating jacket is thenformed over that assembly by wrapping a glass fibre tape over the pipeand cable. The insulated assembly may then in turn be covered with asheath of for example polyethylene.

The known coilable conduit incorporating heating cables is generallyused for frost protection in industrial applications. The known conduitis relatively expensive, for example of the order of £30 per metre. Inaddition it is not easy to work with as coiling and uncoiling of theconduit is difficult if kinking is to be avoided. The making ofconnections both with regard to fluids and electrical power suppliesrequires skilled operators. It is also the case that the cable is not ofcircular outer cross section because the radial thickness of the cableadds to the overall size of the assembly on the side of the assembly atwhich the cable is located. This makes coiling on a roll difficult.Furthermore voids are formed between the pipe and the foil used to holdthe cable on the pipe on either side of the cable and the existence ofsuch voids is generally understood to be undesirable.

Pipes fabricated from plastics material, for example cross-linkedpolyethylene, are well known. Such pipes have certain advantages,notably suitability for use with easy-to-use push-fit couplings. Plasticpipes have not been considered ideal in the past however forapplications in which heating is required because the material fromwhich the plastics pipe is fabricated is resistant to the conduction ofthermal energy through the pipe wall to whatever fluid is carried by thepipe.

Composite pipes are known which comprise a three-layer wall structure,that is an inner plastics tube formed of for example cross linkedpolyethylene, an intermediate metal tube formed of for examplealuminium, and a plastics outer tube formed of for example cross-linkedpolyethylene. The aluminium tube has been incorporated in such pipe toincrease the burst strength and in addition has the advantage that whenthe pipe is bent the pipe remains in the shape into which it is bentrather than springing back, as is the case where no metal tube isincorporated. The relatively thin aluminium tube is sufficient toprovide improved burst strength and the presence of plastics layers oneither side of the aluminium tube makes the composite structurerelatively easy to bend without kinking. Such composite pipe is nowavailable at relatively low prices and proposals have been made for itsuse in both potable water and central heating systems.

The possibility of heating hot water distribution systems in domesticapplications has been previously considered. In a domestic applicationwhere there may be many metres of pipe between a source of hot water anda tap from which that water is dispensed, every time a user wishes todraw hot water the full length of the pipe is filled with hot water andthe energy contained in the water left in;that pipe after the tap isclosed is lost. Calculations have been made which indicate that if suchpipes could be heated economically there would be a significant netsaving in overall energy in many domestic situations. Unfortunately theprovision of a heatable pipe suitable for such applications at aneconomically realistic price has not been considered practical and themounting of heating cable and insulation in situ on previously installedpipework is not a practical proposition.

It is an object of the present invention to provide a heated conduitwhich addresses the problems discussed above.

According to the present invention, there is provided a conduitcomprising a pipe, an electrical heater extending along the length ofthe pipe, and a thermally insulating jacket extending around the pipeand heater, wherein the pipe comprises a plastics inner tube, theplastics inner tube being received within a deformable metal tube suchthat the plastics inner tube and deformable metal tube form a compositedouble-walled structure.

The incorporation in the pipe of a metal tube makes it possible for heatgenerated outside the inner plastics tube to be distributed around thecircumference of the pipe in a manner such that the inherent poorconductivity of the inner plastics tube does not prevent an adequateflow of energy to the contents of the pipe. Given the presence of thedeformable metal tube the thickness of the inner plastics tube can berelatively small, further reducing the resistance to thermal transferpresented by the plastics inner tube.

Preferably, the pipe comprises the plastics inner tube, the metal tube,and an outer plastics tube, the metal tube being sandwiched between theinner and outer plastics tubes, and the heater being in contact with theouter plastics tube Such a three-walled composite structure is robustand resistant to kinking and, despite the presence of the plastics outertube, thermal transfer into the tube from the heater can be sufficientin many applications.

The heater may be in the form of a cable incorporating longitudinallyextending wires between which a heating element is electricallyconnected, the cable being mounted on the pipe. For example, the heatingelement can be in the form of a body of material having a positivetemperature coefficient, the wires being embedded in the body ofmaterial. The cable may be in contact with at least one metal foiladhered to the pipe to further enhance the distribution of thermalenergy from the heater to the pipe. The metal foil can be in the form ofsheet which extends between the cable and the pipe, and/or a sheet whichextends over the cable and onto the pipe on either side of the cable,and/or a metal foil which is wrapped around the cable.

The insulation may comprise first and second preformed layers mounted onthe pipe, the first layer extending around the pipe from side surfacesof the cable and having a thickness substantially the same as thethickness of the cable measured from a lower surface of the cable incontact with the pipe, and the second layer extending over the firstlayer and an upper surface of the cable remote from the pipe. The resultis a structure of substantially circular cross section. A circular crosssection can also be achieved using a single preformed layer ofinsulation mounted on the pipe, the preformed layer being splitlongitudinally to define abutting edges which are stepped to accommodatethe cable in a space formed between the abutting edges

In an alternative arrangement, the heater may comprise a tube ofmaterial having a positive temperature coefficient supported on the pipebetween two metal tubes. One of the tubes of electrically conductivematerial may be defined by the deformable metal tube which forms acomposite structure with the plastics inner tube. The or each plasticstube may be formed from cross-linked polyethylene. The or each plasticstube may be formed from aluminium. The insulation jacket may be formedfrom one or more preformed tubes of foam. One advantage of the use ofpreformed tubes of foam is that differential expansion between the pipeand insulation is easily accommodated by relative movement therebetween.Similarly, foam insulation does not resist bending of the internal pipe.Preferably the internal pipe is formed of material such that it retainsthe shape into which it is bent.

It is to be understood that more than one electrical heater may be usedand that the plastics tube may be, for example, polymeric.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 schematically represents the cross section of a known conduitincorporating a heater;

FIG. 2 schematically represents a cross section of a first embodiment ofthe present invention;

FIG. 3 schematically represents the cross section of a second embodimentof the present invention from which thermal insulation and a coversheath have been removed;

FIG. 4 schematically represents an alternative heater cable supportarrangement to that shown in FIG. 3; and

FIG. 5 schematically represents the cross section of a third embodimentof the present invention.

Referring to FIG. 1, in the known conduit incorporating a heater a pipe1 of for example copper is intended to carry a fluid to be heated by aheating cable 2. The heating cable 2 is secured on the pipe by awrapping film 3. The wrapping film 3 is in turn covered with a fibreglass tape wrapping layer 4 which itself is encased in a polymericsheath 5. The heating cable 2 comprises two longitudinally extendingwires embedded in a body of material having a positive temperaturecoefficient (PTC). The cable also comprises a covering sheath. The twowires of the cable are connected to opposite poles of a power supply andcurrent passes through the PTC material, thereby heating the cable. Heatgenerated in the cable is transferred into the copper pipe 1, thematerial from which the pipe is fabricated ensuring the efficienttransfer of energy from the cable 2 to the fluid within the pipe.

The known structure illustrated in FIG. 1 has been used in industrialapplications but is not suitable for wide spread use in domesticapplications. Great care must be taken to ensure that the copper pipe 1is electrically insulated from the cable in all possible circumstances.Furthermore, the assembly is not circular in cross section because theinsulation 4 and sheath 5 bulge outwards above the cable 2. This makesit difficult for the assembly to be distributed on a roll as thenon-circular assembly is difficult to neatly roll up on to a drum. Afurther problem with this known assembly is that voids are defined oneither side of the cable 2 between the copper pipe 1 and the wrappingfilm 3. These voids affect the thermal characteristics of the assemblyin an undesirable manner.

Referring to FIG. 2, this illustrates a first embodiment of the presentinvention. In the assembly of FIG. 2, an inner tube 6 of cross linkedpolyethylene forms part of a double-wall composite tube the other halfof which is defined by an aluminium tube 7. A heating cable 8 is securedto the outside of the aluminium tube 7 by an adhesive foil 9 whichextends over the cable 8 and on to the aluminium tube 7 on either sideof the cable. The cable 8 comprises two longitudinally extending wires10 and 11 embedded in a body of PTC material. A first layer ofinsulation 12 in the form of a split tube of foam is mounted so as toextend around the aluminium tube 7 from one side surface of the cable 8to the other, the layer of foam 12 having a thickness substantially thesame as the thickness of the cable 8 in the radial direction. A secondlayer of insulation 13 in the form of a split tube of foam is arrangedaround the foam layer 12 and over the cable 8. Thus substantially novoids are formed in the body of insulation adjacent the cable 8. Theinsulation assembly is then covered in a polymeric sheath 14.

With the arrangement of FIG. 2, energy is delivered from the cable 8 tothe adjacent aluminium tube 7 in an efficient manner and is conducted bythe aluminium tube 7 in the circumferential direction away from thecable. That energy is then coupled through the plastics tube 6 to thefluid within the plastics tube 6. Thus, despite the fact that the doublewalled composite structure incorporates a layer of plastics materialwhich is not a good thermal conductor, heat can flow from the cable 8 tothe content of the assembly in an efficient manner. Thus it is notnecessary for the cable 8 to operate at very high temperatures todeliver energy to the content of the assembly.

Referring now to FIG. 3, this illustrates an alternative multi-wallcomposite tube structure to that shown in FIG. 2. The structure shown inFIG. 3 will be enclosed in a double-layer foam insulating jacketenclosed within a polymer sheath just as in the case of the structure ofFIG. 2, but the insulating jacket and sheath are not shown in FIG. 3.

Referring in detail to FIG. 3, in this case the heating cable 8 ismounted on a three-wall composite structure comprising an inner plasticstube 15 of cross link polyethylene, an intermediate aluminium tube 16,and an outer plastics tube 17 of cross linked polyethylene. A metallicfoil 18 is adhered to the plastics tube 17 so as to extend beneath thecable 8. A further metallic foil 19 extends over the cable 8 and isadhered to the foil 18. As a result the cable 8 is housed within ametallic jacket defined by the foils 18 and 19. Such a structureenhances the flow of thermal energy from the cable to the plastics tube17, considerably increasing the heated area of the outer surface of theplastics tube and improving the flow of energy to that surface fromportions of the cable 8 which are not in direct contact with the foil18.

Energy flows from the cable 8 through the foils 18 and 19 to the surfaceof the tube 17. That energy then flows through the tube 17 and isfurther distributed in the circumferential direction by the aluminiumtube 16. The overall efficiency with which thermal energy is deliveredto the content of the inner tube 15 is thus sufficient to be able to usea cable 8 which operates at a relatively low excess temperature ascompared with the desired temperature of the interior of the inner tube15.

FIG. 4 shows an alternative foil arrangement to that shown in FIG. 3. Inthe case of FIG. 4 the cable 8 is wrapped in a foil 20 before beingsecured in position on the outer surface of the plastics tube 17 by afurther foil 21. The two foils 20 and 21 ensure efficient transfer ofthermal energy to the outer plastics tube 17.

In the embodiment of FIG. 2, the fluid-carrying tubes and heating cable8 are insulated by a two-part foam insulator defined by an inner foamtube 12 and an outer foam tube 13. Such a double-walled insulationassembly could be replaced by a single foam tube the inner surface ofwhich is cut back so as to define an inwardly facing void shaped toreceive the cable 8. For example, the tube could be manufactured with aninternal diameter slightly larger than the external diameter of thealuminium tube 7 and define abutting edges one or both of which are cutback to provide the necessary void shaped to receive the projectingcable 8. With either single or double layer foam insulators, preformedfoam tubes can be prepared and delivered in rolled form for positioningon the multi-wall tube and heater assembly during the manufacturingprocess. Relative thermal expansion and contraction of the multi-walledtube assembly during use can be readily accommodated by relativemovement between the multi-walled tube assembly and the foam insulation.

With a conduit such as described in FIGS. 2 to 4, termination is arelatively simple matter. The conduit could be delivered on a roll anddispensed by the installer so as to extend between for example a sourceof domestic hot water and a room in which say two taps are provided todispense that water. The supply conduit could have the outer sheath andinsulation stripped back so as to expose the cable and the end of themulti-walled tube structure. Appropriate connections could then be madeto further conduits of the same type extending to the taps, andappropriate connections could also be made to the heating cables. Forexample, an incoming domestic water supply conduit could be connected toa manifold with conduits extending from the manifold to each tap. Themanifold could incorporate a simple electrical connector assembly towhich cables mounted on the incoming and outgoing conduits could beeasily connected.

Referring to FIG. 5, an alternative conduit in accordance with thepresent invention is illustrated which comprises an inner plastics tube22 of for example cross linked polyethylene, an aluminium tube 23, alayer 24 of a material with a positive temperature coefficient, afurther aluminium tube 25, and an outer plastics tube 26 of for examplecross-linked polyethylene. The outer plastics layer may be covered in aninsulating layer 27 which in turn is housed within a polymer sheet 28.In this multi-walled composite structure, the heater is defined by thealuminium tubes 23 and 25 and the PTC material layer 24 sandwichedtherebetween. The tubes 23 and 25 are connected to opposite poles of apower supply such that current flows between them through the PTCmaterial layer 24. Thermal energy is coupled in a highly efficientmanner through the aluminium tube 23 and the plastics tube 22 to thecontent of the overall assembly.

Termination of a conduit such as that described in FIG. 5 will requireseparate connections to be made to the aluminium tubes 23 and 25. Tofacilitate such connections, it will be desirable to make the tube 25 ina manner which facilitates easy stripping back of that layer from theend of the assembly. Fluid connection could then be made to thedouble-walled structure defined by the inner plastics layer 22 and theinner aluminium tube 23. The inner aluminium tube 23 could be connectedso as to define an earthed return of the electrical power supply system.

1. A conduit comprising: a pipe; an electrical heater extending alongthe length of the pipe; a thermally insulating jacket extending aroundthe pipe and heater; wherein the pipe comprises a plastics inner tube,the plastics inner tube being received within a deformable metal tubesuch that the plastics inner tube and deformable metal tube form acomposite double-walled structure; wherein the heater is in the form ofa cable incorporating longitudinally extending wires between which aheating element is electrically connected, the cable being mounted onthe pipe; and wherein the insulating jacket comprises first and secondpreformed layers mounted on the pipe, the first layer extending aroundthe pipe from side surfaces of the cable and having a thicknesssubstantially the same as the thickness of the cable measured from alower surface of the cable in contact with the pipe, and the secondlayer extending over the first layer and an upper surface of the cableremote from the pipe.
 2. A conduit according to claim 1, wherein theinsulation comprises a single preformed layer mounted on the pipe, thepreformed layer being split longitudinally to define abutting edgeswhich are stepped to accommodate the cable in the space formed betweenthe abutting edges.
 3. A conduit according to claim 1, wherein the pipecomprises the plastics inner tube, the metal tube, and an outer plasticstube, the metal tube being sandwiched between the inner and outerplastics tubes, and the heater being in contact with the outer plasticstube.
 4. A conduit according to claim 1, wherein the heating element isin the form of a body of material having a positive temperaturecoefficient and wires extending longitudinally therethrough, the wiresbeing embedded in the body of material.
 5. A conduit according to claim1, wherein the heater is in contact with at least one metal foil adheredto the pipe.
 6. A conduit according to claim 5, comprising a metal foilwhich extends between the heater and the pipe.
 7. A conduit according toclaim 6, comprising a metal foil which extends over the heater and on tothe pipe on either side of the heater.
 8. A conduit according to claim 1comprising a metal foil which is wrapped around the cable.
 9. A conduitaccording to claim 1, wherein the or each plastics tube is formed fromcross-linked polyethylene.
 10. A conduit according to claim 1, whereinthe or each metal tube is formed from aluminium.
 11. A conduit accordingto claim 1, wherein the insulating jacket is formed from one or morepreformed tubes of foam.
 12. A conduit according to claim 1, wherein thepipe is formed from materials such that if bent it retains the shapeinto which it is bent.
 13. A conduit comprising; a pipe; an electricalheater extending along the length of the pipe; a thermally insulatingjacket extending around the pipe and heater; wherein the pipe comprisesa plastics inner tube, the plastics inner tube being received within adeformable metal tube such that the plastics inner tube and deformablemetal tube form a composite double-walled structure; and wherein theheater comprises a tube of material having a positive temperaturecoefficient supported on the pipe between two metal tubes.
 14. A conduitaccording to claim 13, wherein one of the said tubes of electricallyconductive material is defined by the said deformable metal tube inwhich the plastics inner tube is received.
 15. A conduit according toclaim 13, wherein the pipe comprises the plastics inner tube, the metaltube, and an outer plastics tube, the metal tube being sandwichedbetween the inner and outer plastics tubes, and the heater being incontact with the outer plastics tube.
 16. A conduit according to claim13, wherein the heater is in contact with at least one metal foiladhered to the pipe.
 17. A conduit according to claim 16, comprising ametal foil which extends between the heater and the pipe.
 18. A conduitaccording to claim 17, comprising a metal foil which extends over theheater and on to the pipe on either side of the heater.
 19. A conduitaccording to claim 13, wherein the or each plastics tube is formed fromcross-linked polyethylene.
 20. A conduit according to claim 13, whereinthe or each metal tube is formed from aluminium.
 21. A conduit accordingto claim 13, wherein the insulating jacket is formed from one or morepreformed tubes of foam.
 22. A conduit according to claim 13, whereinthe pipe is formed from materials such that if bent it retains the shapeinto which it is bent.
 23. A heated conduit comprising: a pipecomprising: a polymeric inner tube; a deformable metal tube; and anouter polymeric tube, the metal tube being sandwiched between the innerand outer polymeric tubes and the polymeric inner tube being receivedwithin said deformable metal tube such that the polymeric inner tube anddeformable metal tube form a composite double-walled structure; anelectrical heater extending along the length of the pipe and in contactwith the outer polymeric tube; and a thermally insulating jacketextending around the pipe and heater, said insulating jacket comprisingfirst and second preformed layers mounted on the pipe, the first layerextending around the pipe from side surfaces of the heater and having athickness substantially the same as the thickness of the heater, and thesecond layer extending over the first layer and a radially outer surfaceof the heater.
 24. A conduit according to claim 23, wherein the heateris in the form of a cable incorporating longitudinally extending wiresbetween which a heating element is electrically connected, the cablebeing mounted on the pipe.
 25. A conduit according to claim 23, whereinthe heater includes a heating element in the form of a body of materialhaving a positive temperature coefficient and wires extendinglongitudinally therethrough, the wires being embedded in the body ofmaterial.
 26. A conduit according to claim 23, wherein the heater is incontact with at least one metal foil adhered to the pipe.
 27. A conduitaccording to claim 26, comprising a metal foil which extends between theheater and the pipe.
 28. A conduit according to claim 27, comprising ametal foil which extends over the heater and on to the pipe on eitherside of the heater.
 29. A conduit according to claim 26 comprising ametal foil which is wrapped around the cable.
 30. A conduit according toclaim 23, wherein the or each polymeric tube is formed from cross-linkedpolyethylene.
 31. A conduit according to claim 23, wherein the or eachmetal tube is formed from aluminum.
 32. A conduit according to claim 23,wherein the insulating jacket is formed from foam.
 33. A conduitaccording to claim 23, wherein the pipe is formed from materials suchthat if bent it retains the shape into which it is bent.